1. Field of the Invention
The invention relates to an apparatus and method for monitoring a steel decarburization process. In particular, the invention relates to an apparatus and method for monitoring spatial and spectral changes of a steel decarburization furnace flame by flame emission imaging spectroscopy carried out from a location remote from the furnace flame.
2. Description of the Related Art
During the manufacturing of high-quality stainless steel, carbon impurities are removed by "blowing" molten steel in a furnace such as an argon oxygen decarburization (AOD) furnace. The decarburization process must be closely monitored to determine the endpoint of the process. If the decarburization process is ended too soon, carbon impurities remain in the steel. If the decarburization process is carried out for too long, expensive metal additives (e.g. Cr, Ni, Mo, etc.) are lost.
Typically, decarburization processes are monitored by thermal characterization and complex mathematical modeling. When a decarburization process is thought to be complete, a sample of the steel batch is removed from the furnace and tested in a laboratory to determine if an acceptable level of unwanted carbon has been removed and if an acceptable level of specialty metals has been retained. Steel batches are often tested for composition and purity several times during the course of a blow. This method of monitoring chemical composition is disadvantageous in several ways. First, it relies on an indirect measurement of the steel's chemical composition, using theoretical mathematical models and physical temperature measurements to extrapolate a measure of carbon content. Second, it relies on disposable thermocouples for the collection of temperature data. Because this is cost, time and labor intensive, it limits batch testing to periodic intervals (often with unacceptable frequency). In short, the current method of monitoring decarburization has proven insufficient. Prediction of the batch carbon content has been limited, thus multiple sampling and laboratory testing has become common practice. As expected, this can lead to economic losses through unnecessary testing or, more seriously, through the destruction of a steel batch (or at least its very expensive metal additives).
Various less typical methods of monitoring metallurgical processes are described, for example, in the following U.S. patents incorporated herein by reference: U.S. Pat. No. 2,803,987 to Galey, U.S. Pat. No. 3,181,343 to Fillon, U.S. Pat. No. 3,329,495 to Ohta et al, U.S. Pat. No. 3,594,155 to Ramachandran, U.S. Pat. No. 3,720,404 to Carlson et al, U.S. Pat. No. 3,741,557 to Harbaugh et al, U.S. Pat. No. 4,251,270 to Hoshi et al, U.S. Pat. No. 4,416,691 to Narita et al, U.S. Pat. No. 5,603,746 to Sharan and U.S. Pat. No. 5,522,915 to Ono et al.
Visual observation of flames has also been used in metallurgy to determine the endpoint of a process. This method has the disadvantage of being highly prone to human error.
U.S. Pat. No. 5,125,963 to Alden et al describes the use of optical spectroscopy in the visible region of the spectrum to monitor metallurgical processes and mentions that optical spectroscopy can be used to measure the intensities of carbon compounds during iron and steel production.
Because of the dynamic behavior of a furnace flame, optical spectroscopy using non-imaging optics and a one-dimensional detection scheme to measure spectral emission at a single point in the flame is inadequate to accurately monitor decarburization. Spectra collected at a single point in the flame are likely to show dramatic changes in baseline (background) intensity as a function of the flame's ever-changing spatial characteristics. Baseline fluctuations may be eliminated by taking an integrated measure of the spectral emission over the entire flame. However, doing so also reduces or eliminates the signal of the comparatively smaller emission bands of interest.